In long haul fibre optic communications systems it is important to be able to monitor the transmission properties of the system. Monitoring can be used to locate faults in the fibre or in repeaters or amplifiers. It is also desirable to be able to carry out monitoring whilst the communications system is in service. Various schemes have been proposed.
GB2268017 describes a number of techniques based on the provision of a supervisory signal and a high loss loopback system in which a small proportion of the composite traffic and supervisory signal is looped back at every repeater in the transmission link. The supervisory signal may comprise a pseudo random signal, a reference signal with frequency sweep or a short pulse, and is transmitted using a sub-carrier over a single wavelength traffic signal. Information from the loopback signal is then obtained by detecting correlations, beat frequencies or simple time delays, respectively.
One problem with the techniques described in GB2268017 is the very low signal-to-noise ratio (SNR) associated with the returning signal, wherein the loopback supervisory component must be extracted from the background traffic signal and noise. An associated problem is the slow speed at which information on faults in the transmission system can be collated. For example, in the case of the short pulse supervisory signal, detection relies on a photodiode followed by a simple electrical bandpass filter for discrimination and the filtered signal is simply displayed on an oscilloscope.
U.S. Pat. No. 5,825,515 also discloses a method based on a high loss loopbacK system with a pseudo random supervisory signal. A low bit rate supervisory Pseudo Random Bit Sequence (PRBS) signal, which is Binary Phase Shift Keyed (BPSK) encoded and optically Intensity Modulation (IM) encoded is transmitted using a sub-carrier over a single wavelength traffic signal. A small proportion of the composite traffic and supervisory signal is looped back at every repeater and the supervisory signal is deconvolved from the noise by integration and correlation to yield a supervisory signal level from each repeater. However, using this method in a typical system means an in-service measurement time of the order of 4 hours.
The reason for the long measurement time is again the very low signal to noise ratio of the returned pulses. As a result, long integration times are required to extract the signal with any degree of confidence. Although a data cancellation scheme is employed in the system of U.S. Pat. No. 5,825,515, it is not possible to eliminate much of the noise generated in the PIN diode in the receiver. This problem is addressed U.S. Pat. No. 5,969,833, in which the supervisory signal is transmitted over a separate wavelength. This scheme also reduces the traffic signal penalty due to the supervisory modulation. However, both patents advocate the use of PRBS sequences, which must be de-convolved at the supervisory receiver in order to measure the loop loss to a given repeater. A separate de-convolution process is required for every repeater, which is expensive when many repeaters are present in a system.
The present invention aims to provide a simple, low-cost solution for the in-service monitoring of fibre optic transmission systems, and moreover a solution which delivers the required information in a satisfactory time period.